This invention relates to a method and apparatus for the beneficiating of ores and is particularly directed to a method and apparatus for classifying ores by size, shape and density while concurrently leaching values from said ores.
Current practice in leaching of gold and other precious metals from an ore involves crushing and grinding the ore to reduce its largest particle size to the point where intimate mixing of a leaching solution, such as sodium cyanide in water, with the particles will cause the gold or precious metal in the particles to dissolve into the solution from which the gold or precious metals are extracted at a later stage in a process plant.
In gold mills currently in use ore which has been diluted in water during the conventional grinding and classification process in grinding mills is thickened in a rake thickener or other device to about 50% solids by weight and then pumped as a slurry to a series of tanks where leaching agents are added. By keeping the slurry agitated with paddle mixers or other mechanical, pneumatic or hydraulic means, intimate mixing of the particles and leaching in solution takes place. Usually a series of tanks are used, one overflowing to the next, such that a leaching time of 12 to 72 hours takes place.
Recently, the high cost of processing low-grade gold and precious metals ores has led to cheaper methods of leaching such as "heap leaching" and "vat leaching".
Heaping leaching involves crushing, but rarely grinding, of an ore and laying the ore by mechanical means as a layer onto a pad on the ground or other prepared surface. A leaching agent is sprayed onto the layer of ore, after which it percolates through the pile, dissolving some of the gold or precious metals. The solution is then collected by drainage underneath and around the pile to a sump, where the solution may be recirculated to the top of the ore pile as often as is necessary to leach out the gold or precious metals, or pumped to a plant to have the gold or precious metals extracted from the solution.
Vat leaching is similar to heap leaching, except that the ore is sometimes ground and placed in a pit or like tank instead of a pile such that the ore is immersed for a more effective soaking to improve contact of the leaching solution with the ore.
Heap leaching or vat leaching is not nearly as efficient as conventional leaching in agitated tanks because the larger particle sizes and lack of mixing does not allow an intimate contact to be made between the leaching agent and the gold or precious metals. Leaching times are typically 10 to 15 days for vat leaching and weeks or months for heap leaching. In addition, both methods are batch processes, requiring adding and removing the ore from the leach area by mechanical means.
It is known to beneficiate ores by classifying solids in slurries using beds fluidized by a countercurrent flow of a liquid or gas medium. Known method, such as for separating bitumen from oil sands, use a continuous flow of rising fluid, usually water, countercurrent to descending solids and effect a separation according to size, shape and density. The product recovered usually is substantially diluted by the volume of separating medium required and must be thickened.